FIG. 16 is a partially exploded perspective view showing the structure of a prior art polymer electrolyte fuel cell.
As illustrated in FIG. 16, the polymer electrolyte fuel cell 100 is composed of cells 10 stacked therein.
Although not shown in FIG. 16, a current collecting plate, an insulating plate and an end plate are mounted on the outermost cells located at both ends of the cell stack 10. The cells 10 are fastened with fastening bolts that pass through bolt holes 4 and nuts (not shown).
Each cell 10 is composed of an MEA-gasket assembly 1 that is sandwiched between a pair of separators, i.e., an anode separator 2 and a cathode separator 3 (herein, these are generically called “separators”).
The MEA-gasket assembly 1 is formed such that a polymer electrolyte membrane constituting the peripheral region of an MEA 5 is sandwiched between a pair of gaskets 60 made of fluorocarbon rubber. Specifically, the MEA 5 is comprised of the polymer electrolyte membrane having main faces on each of which a catalyst layer and a gas diffusing layer 5C are formed. In the MEA 5, the polymer electrolyte membrane projectingly extends outwardly from the catalyst layers and the gas diffusing layers 5C. The gaskets 60 are joined so as to hold the extending portion of the polymer electrolyte membrane between. Accordingly, the gas diffusing layers 5C are exposed through the central openings, respectively, of the gaskets 60 of the MEA-gasket assembly 1.
In the peripheral regions of the separators 2, 3 and the MEA-gasket assembly 1, reducing gas manifold holes 12, 22, 32 and oxidizing gas manifold holes 13, 23, 33 are provided, such that a pair of manifolds through which reducing gas flows and a pair of manifolds through which oxidizing gas flows are formed, when the cells 10 are assembled. Water manifold holes 14, 24, 34 are also provided so as to form a pair of manifolds through which water flows.
In the inner main face of the anode separator 2, a groove-like reducing gas passage 21 is formed so as to connect the pair of reducing gas manifold holes 22,22 to each other.
In the inner main face of the cathode separator 3, a groove-like oxidizing gas passage 31 is provided so as to connect the pair of oxidizing gas manifold holes 33,33 to each other.
Although not shown in FIG. 16, the outer main faces (backside) of the separators 2, 3 are respectively provided with a groove-like water passage that connects the pair of water manifold holes 24 (34) to each other, similarly to the reducing gas passage 21 and the oxidizing gas passage 31.
When the cells 10 are stacked, the oxidizing gas, reducing gas and water manifold holes are respectively aligned, thereby forming a pair of oxidizing gas manifolds, a pair of reducing gas manifolds and a pair of water manifolds. The flowing routes of the oxidizing gas, reducing gas and water are respectively formed such that each fluid flows from one manifold (i.e., the supply side manifold) to the other manifold (i.e., the discharge side manifold), while diverging to the passage 21, passage 31 or water passage (not shown) formed on the separators. One face of the exposed portion of the MEA located within the central part of the MEA-gasket assembly 1 is exposed to the oxidizing gas flowing in the oxidizing gas passage 21, and the other face is being exposed to the reducing gas flowing in the reducing gas passage 31, so that an electrochemical reaction occurs. Since water flows in the back faces of the separators 2, 3, that is, between the adjacent cells 10, the polymer electrolyte fuel cell 100 can be kept to a specified temperature appropriate for the electrochemical reaction by the heat transfer ability of the water.
FIG. 17 is an enlarged perspective view showing a section taken along line XVII-XVII of FIG. 16. As illustrated in FIG. 17, a diverging point leading to the reducing gas passage 21 is formed between a reducing gas manifold hole 12 of the MEA-gasket assembly 1 and a reducing gas manifold hole 22 of the anode separator 2. The reducing gas flows into the reducing gas passage 21 in the direction indicated by solid arrow in FIG. 17. The portion of the MEA-gasket assembly 1 located in the diverging point that leads to the reducing gas passage 21 is joined to the cathode separator 3 only and is not pressed from the side of the anode separator 2. In addition, the peripheral region of the MEA-gasket assembly 1 is easily deformed because it is constituted by the gaskets 60 that are made of an elastic material. Accordingly, the sealing performance is poor in the region between the cathode separator 3 and the MEA-gasket assembly 1 and therefore there is a possibility that the reducing gas penetrates from the diverging point into the space between the cathode separator 3 and the MEA-gasket assembly 1 as indicated by dashed arrow of FIG. 17. Alternatively, the oxidizing gas flowing in the oxidizing gas passage 31 defined by the cathode separator 3 and the MEA-gasket assembly 1 may leak to the diverging point. As a result, mixing of the oxidizing gas and the reducing gas, that is the so-called cross-leak phenomenon, occurs, which leads to a possibility of a decrease in the performance of the polymer electrolyte fuel cell 100 and damage to the inside of the polymer electrolyte fuel cell 100. Although not shown in the drawings, the same may happen to the diverging point that leads to the oxidizing gas passage 31, the diverging point being located between an oxidizing gas manifold hole 33 of the cathode separator 3 and an oxidizing gas manifold hole 13 of the MEA-gasket assembly 1. Specifically, since the sealing performance deteriorates in the region between the anode separator 2 and the MEA-gasket assembly 1, the oxidizing gas may penetrate into the space between the anode separator 2 and the MEA-gasket assembly 1, or alternatively, the reducing gas leaks, causing the cross-leak phenomenon of the oxidizing gas and the reducing gas.
As attempts to solve this problem, there have been proposed fuel cells capable of restraining the cross-leak at the diverging points by providing a covering material between the separators 2, 3 and the MEA-gasket assembly 1 (see Patent Document 1) or by forming the oxidizing gas passage 31 and reducing gas passage 21 of the separators 2, 3 in the aforesaid area from holes that pass through the separators 2, 3 (see Patent Document 2).
In addition, the flows of the reducing gas and oxidizing gas between the pair of reducing gas manifold holes 22 and between the pair of oxidizing gas manifold holes 33, which pass through the annular gap around the polymer electrolyte membrane 5A between the inner peripheral edges of the gaskets 60 and the outer peripheral edges of the gas diffusing layers 5C (i.e., the annular gap located around the outer peripheries of the gas diffusing layers 5C), cause a decrease in the oxidizing gas/reducing gas utilization factor and a decrease in the efficiency of the polymer electrolyte fuel cell 100.
Patent Document 1: Publication of Examined Application No. 1-60899
Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2002-83614